1. Field of the Invention
The present invention relates to an optical recording media comprising information layers stacked in the light incident direction, at least one of the information layers including a rewritable recording film or a write-once recording film, and also relates to an optical recording-reproducing apparatus recording to and reproducing from the optical recording media.
2. Description of the Related Art
(Principle of Phase-change Optical Recording Media)
The phase-change optical recording media, which uses a phase-change optical recording film that brings about reversible phase change between the crystalline and amorphous phases by irradiation with a light beam, operates according to the following principle. Recording is performed by heating the region irradiated with a light beam to temperatures higher than the melting point of the film so as to melt that region, followed by rapidly cooling the molten region so as to change atomic arrangement in the cooled region to form an amorphous phase. Erasure is performed by retaining the temperature of a region irradiated with a light beam for at least a prescribed time such that the temperature falls within a range between a level not higher than the melting point and not lower than the crystallizing temperature. In this stage, in the case where the initial state is crystalline, the crystalline phase is maintained, and in the case where the initial state is amorphous, the amorphous phase is changed into the crystalline phase. Reading is performed by converting the intensity of the reflected light into the intensity of an electrical signal by utilizing the phenomenon that the intensity of the reflected light from the amorphous region differs from the intensity of the reflected light from the crystalline region, followed by subjecting the electrical signal to analog-to-digital conversion. In reading, it is also possible to utilize phase difference between the light reflected from the amorphous region and the light reflected from the crystalline region.
It should noted that write and read can be performed by utilizing phase change between a Metastable crystalline phase such as martensite and a stable crystalline phase or phase change between metastable crystalline phases as well as the phase change between the crystalline phase and the amorphous phase noted above.
(Approaches for Improving the Recording Density)
For improving the amount of information that can be recorded in a single recording media, i.e., the recording capacity, it is conceivable to employ the two approaches given below.
A first approach is to reduce the pitch of the recording marks in the track direction, i.e., the so-called bit pitch. However, when the bit pitch is significantly reduced, it will become smaller than the size of a read beam. In such a case, two recording marks may be included temporarily in a read beam spot. Where the recording marks are sufficiently separated from each other, the read signals are greatly modulated so as to make it possible to obtain signals with high amplitude. However, where the recording marks are close to each other, the obtained signals have lowered amplitude, with the result that errors tend to occur in the stage of converting the signals into digital data.
The other approach is to reduce the track pitch. This approach makes it possible to improve the recording density without being affected significantly by lowering in the signal intensity that is caused by the bit pitch reduction noted above. In this approach, however, a so-called cross-erase problem is caused. Specifically, if the track pitch is made substantially equal to or smaller than a light beam size, the information in a certain track deteriorates when writing or erasure is performed on the adjacent track.
The cross-erase is caused partly because the mark on the track in question is irradiated directly with the edge of the laser beam on the adjacent track, and partly because heat generated in recording flows from the adjacent track so as to raise the temperature of the mark on the track in question and, thus, to deform the mark. In order to improve the recording density of the phase-change optical recording media, it is necessary to overcome the problems pointed out above. Also, in order to suppress the probability of read errors for small recording marks to a low level, it is desirable that the recording marks be formed to have an even contour so as to suppress noise components as much as possible.
It is expected that a light beam with a shorter wavelength greatly contributes to the approaches of reducing the bit pitch and the track pitch. At present, a blue-violet semiconductor laser (LD) with the shortest wavelength in the visible region has been put to a practical use. It should be noted that a light source in an ultraviolet region is used in the manufacture of a master of the optical disc, i.e., in the so-called mastering process, but a commercially available ultraviolet semiconductor laser has not yet been realized. However, polycarbonate generally used for the substrate significantly absorbs ultraviolet light with a shorter wavelength than the visible light. The polymer material generally tends to absorb light in the ultraviolet region. Thus, it is necessary to use a substrate other than the conventional one in order to satisfy the requirement for use in the ultraviolet region. However, a substrate material suitable to the ultraviolet region has not yet been found. This is also the case with not only the substrate but also the materials for optical parts used in an optical recording-reproducing apparatus. Therefore, it is considered advantageous to employ a multilayered optical recording media described in the following for further improvement in the recording capacity.
(Improvement in Recording Capacity by Use of a Multilayered Optical Recording Media)
Another approach for improving the recording capacity is to stack information layers each including a phase-change optical recording film. See, for example, Japanese Patent Disclosure (Kokai) No. 2000-322770. The recording media designed such that two information layers are stacked and the information is written to and read from one side is called a single-sided, dual-layer disc or is simply called a dual-layer disc. In the single-sided, dual-layer disc, it is necessary for the information layer close to the light incident side, which is hereinafter called L0, to have at least 50% of transmittance. This is because it is important to prevent the light from being attenuated excessively in the information layer L0 close to the light incident side in accessing to the information layer remote from the light incident side, which is hereinafter referred to as L1. In order to achieve the requirement, it is necessary for the phase-change optical recording film included in the L0 information layer to be very thin, i.e., not thicker than 10 nm. In the case of such a thin recording film, the retention time required for crystallization is prolonged, with the result that the recording marks fail to be erased completely at a normal recording speed, bringing about lowering of erasure rate. As a measure for overcoming the difficulty, it is known that substitution of a part of the GeSbTe recording film with Sn or with Bi, In, Sn and Pb is effective. On the other hand, it is necessary for the L1 layer to perform recording and erasure with the laser beam with intensity substantially halved by the L0 layer, the L1 layer is required to exhibit high sensitivity.
A technique relating to a triple-layer media or a quadruple-layer media having increased information layers is also known. See, for example, Iida et al., The Technical Report of the Proceeding of the Institute of Electronics, Information and Communication engineers, CPM 2000-96 (2000-09). Also in the multilayered optical recording media, the information layers are defined as L0, L1, L2, and L3 as viewed from the light incident side. The document discloses a method of manufacturing a triple-layer or quadruple-layer ROM media comprising steps of: depositing the L0 layer on a substrate, forming an interlayer separating layer made of a resin by a so-called 2P (Photo-Polymerization) process, transferring grooves or pits onto the interlayer separating layer with a plastic stamper, stripping the plastic stamper, and depositing the L1 layer on the interlayer separating layer. However, since the method requires the process of stripping the plastic stamper from the interlayer separating layer, it is difficult to manufacture even the dual-layer media. Naturally, it is very difficult to commercially manufacture a multilayered media at low cost with high stability.
Another single-sided, dual-layer media having a thin cover layer mounted on the light incident side of the information layer is also proposed on the prerequisite that a blue-violet light source and an optical pickup including an objective lens having a high numerical aperture NA (a high NA type objective lens) are employed. In the single-sided, dual-layer media of this type, the thickness of the cover layer on the light incident side is set to about 75 μm and the thickness of the interlayer separating layer is set to about 25 μm. However, since the characteristics of the particular single-sided, dual-layer media are markedly affected by thickness distribution of the interlayer separating layer and the cover layer, it is difficult to control the characteristics. The method of manufacturing the particular single-sided, dual-layer media also includes steps of: forming an interlayer separating layer by the so-called 2P process, transferring grooves or pits on the interlayer separating layer with a plastic stamper, and stripping the plastic stamper. Thus, it is difficult to commercially manufacture with high stability and at low cost the next-generation single-sided, dual layer media with a recording density higher than that of DVD available at present by using a thin cover layer and by the manufacturing method including the step of stripping the plastic stamper. Further, even if commercial manufacture of a triple-layer or quadruple-layer media is attempted by application of the technique described above, it is estimated substantially impossible to achieve stable and low-cost manufacture.
On the other hand, there is proposed a method of manufacturing a next-generation rewritable media comprising steps of: depositing an information layer on each of two rigid substrates having a thickness of about 0.6 mm, and bonding the two substrates, like the method of manufacturing the current dual-layer media, i.e., a so-called DVD-9. Since the particular method does not include the process of stripping the plastic stamper and does not necessitate the use of a thin cover layer, it is possible to commercially manufacture the single-sided, dual layer media with high stability and at low cost. Therefore, it is desirable for the particular manufacturing method to be applicable to the manufacture of the triple-layer or quadruple-layer media.
(Interlayer Cross-talk)
In order to sufficiently focus the laser beam on each information layer and to suppress loss of the laser beam to a minimum level in the triple-layer or quadruple-layer media, it is necessary to make the interlayer separating layer thinner than that in the dual-layer media. In this case, during read operation from a certain information layer, the read signals may be significantly affected by the so-called interlayer cross-talk (XT), which is caused by leakage signals or noises from another information layer from which data is not being read, i.e., the information layer not in reproducing. Therefore, it is necessary to develop a technique for avoiding the interlayer cross-talk.
(Methods for High-speed Recording)
High-speed recording is also required for the phase-change optical recording media. For example, if the recording can be performed in a time shorter than the actual viewing time, it is possible to easily realize the so-called “time-shift function” that the previous images can be viewed in the copying stage of the distributed recording media or during the real-time recording of the broadcasting images. However, one of the factors for inhibiting the high-speed recording in phase-change recording is a problem that the data fails to be erased completely when crystallization is performed in the overwrite stage with a laser beam at a relatively low erasure level, i.e., the problem of the insufficient erasure rate. Since a recording mark passes through the laser spot at a high speed, it is difficult to retain the recording mark for a sufficiently long time in temperature range within which the crystallization can be occurred, with the result that the data fails to be erased completely. Therefore, uniformity of the shape of the recording marks and the recording film itself becomes more important in view of the erasure operation, too. For improving the uniformity of the mark shape, the uniformity of the initialized state, i.e., the crystalline state, becomes very important.
(Film Design for Phase-change Optical Recording Media)
In the phase-change optical recording media, an amorphous mark or data is written in a desired portion of the recording film by irradiating that portion with a pulsed laser beam. In contrast, the recording film is irradiated with a laser beam so as to crystallize the amorphous mark, thereby erasing the data. In the former stage, an amorphous mark is formed by rapidly cooling the laser beam-irradiated portion, and in the latter stage, the amorphous portion is crystallized by gradually cooling the laser beam-irradiated portion. Also, the recording and erasure can be performed with a lower laser power when the absorbance in the recording film is high. In contrast, a higher laser power is required for recording and erasure when the absorbance in the recording film is low. The absorbance in the recording film is determined by the optical characteristics of the recording media formed of a multi-layered film. What is also important is the thermal design relating to the film structure as to, for example, whether a rapid cooling structure is established, even if the absorbance is the same. Thus, in the film design for the phase-change optical recording media, the optical design and the thermal design are mainly taken into consideration. For the optical design, the optical characteristics of each thin film are needed. Also, for the thermal design, the thermal properties, including the melting point, the latent heat of melting, and the crystallization temperature, are needed for each thin film.
As described above, in the case of the single-sided, dual-layer media, it is necessary for the L0 layer on the light incident side to exhibit a transmittance not lower than 50%. As a result, it is necessary for the L0 layer to be made markedly thin, i.e., the thickness of the L0 layer is required to be 5 to 7 nm. Also, the thickness of the L1 layer is not significantly great, i.e., the thickness of the L1 layer is required to be 10 nm or less. The multilayered optical recording media including at least three information layers, which is available at present, is a ROM or a write-once media (R media). When it comes to the rewritable media, a multilayered optical recording media has not yet exhibited practical characteristics. In the case of a single-sided, triple-layer optical recording media, it is necessary for each of the L0 and L1 information layer on the light incident side to exhibit a transmittance of at least 70%. In the case of a single-sided, quadruple-layer media, it is necessary for each of the L0, L1 and L2 information layer on the light incident side to exhibit a transmittance of at least 80%. Under the circumstances, in the multilayered media including at least three information layers, the phase-change recording film included in each of the information layers on the light incident side is considered to play a more important role, compared with that included in the single-sided, dual-layer media. In addition, it is necessary to consider the problem of interlayer cross-talk in the multilayered media, which is not considered in the single-layer media. Therefore, the optical design and the thermal design of the media including the selection of the materials for the recording film and the interface film become more important.